Add a setting for using the platform dialog

Note: it's off by default on all platforms now, based on the
discussion here:

https://krita-artists.org/t/option-for-kritas-color-selector-dialog-on-mac/33339/5

Note: not for 5.0 -- because it changes the default behaviour
of Krita on macOS.

CCMAIL:kimageshop@kde.org